This application pertains to the electrostatic painting, and in particular to the electrostatic painting of non-conductive plastics.
In recent years, the automotive industry has increased its use of plastic materials for exterior car body panels and trim parts. The predominant reasons are weight-reduction and the availability of more sophisticated high impact strength plastics, such as Noryl GTX(copyright) resin, a poly(arylene ether)/polyamide composition. To a large extent, the future success of plastics for car body panels will depend on their ability to be painted xe2x80x9con-linexe2x80x9d in the assembly plant with a high quality appearance similar to painted metal car body panels.
When a metal body panel is painted, it is relatively simple to maintain the metal at ground potential. In the electrostatic painting process, particles (paint droplets) are charged by an electrode in the spray head, and a charged spray cloud from the spray head is attracted to the metal surface by the high voltage difference. This process greatly reduces over-spray and produces a high quality surface on the painted metal parts. For these reasons, and others, electrostatic spray painting techniques are the methods of choice in the automotive industry for spray painting exterior body panels made of sheet metal.
Difficulties arise when using electrostatic spray techniques for painting plastic substrates. The problem is particularly difficult when the objective is to paint plastics with the same high quality and appearance as metal parts using electrostatic paint spray equipment. In order to electrostatically spray paint plastic substrates, a number of technical problems must be overcome. For instance, electrostatic charges accumulate on the surface of a plastic substrate during the electrostatic spray painting process. The charges that accumulate do not dissipate as readily as with metals. This accumulation of charges reduces the potential between the spray head and the substrate, leading to weaker electrical forces on the charged paint droplets. The accumulated charges on the substrate surface also cause an opposing electrical field that repels air-borne paint particles, and tend to produce a non-uniform field across the surface. These phenomena produce a self-limiting effect of yielding less paint deposition and producing less uniformity in the build-up of the paint film when compared with painting metal substrates. In addition, some plastics have retained charges that may continue to exist for long time periods after the paint has been sprayed, making the painted surface more vulnerable to dust attraction. As a result of these problems, it has been difficult to achieve a high quality paint coat by electrostatic painting of plastics. The problem is particularly difficult when the objective is to apply uniform paint coats to plastic panels having complex three-dimensional shapes.
Conductive plastic substrate materials comprised of plastic and conductive filler, can be painted by electrostatic spray techniques. The conductive filler alleviates the build up of surface charges and the resulting low deposition and non-uniform build up of paint films on plastics. Conductive plastic substrates do have drawbacks however. The base color is typically black and hence it is difficult to achieve good paint coverage, especially with light colors. More significant, however, is that many conductive fillers diminish the desirable physical properties, such as ductility, of the finished part.
Another approach has been to develop electrically conductive primers which are air-sprayed onto the plastic substrate prior to electrostatically spraying on the finished paint film. Conductive primers can reduce the problems of accumulated electrostatic charges, low paint film build-up and non-uniform conductivity and film thickness, but in order to effectively use such conductive primers, certain technical problems first must be overcome. Such problems have been controlling the surface smoothness of the primer and achieving good adhesion to the polymer. The conductive primer needs a good level of surface conductivity along with humidity insensitivity, uniformity of conductivity across the primer surface, and durability. If surface conductivity is too low, non-uniform build-up of the paint film can result. Surface conductivity, as measured in terms of xe2x80x9cresistivityxe2x80x9d (ohms), should be reasonably insensitive to humidity; otherwise non-uniformities in conductivity and in the paint film build-up are produced. Coating thickness can alter the uniformity of surface conductivity. When coating thickness varies as the primer is applied it is difficult to achieve uniform surface conductivity.
Generally speaking, the use of conductive primers for plastic substrate panels in the automotive industry has not been successful in economically producing a high quality finish. Because of non-uniform conductivity and primer film thickness, this priming technique has resulted in a generally poor appearance of the finished paint film. A non-uniform primer, even though an undercoat in the process, can cause a poor appearance of the finished exterior paint coat. It is difficult to produce a uniform paint film thickness with a primer applied by non-electrostatic air spray techniques, followed by air spraying a charged-particle paint film.
As a result of the above mentioned problems, the technique of using conductive primers has resulted in a high scrap rate and increased production time. The current method of priming plastic parts for electrostatic paint spraying involves the additional step of either shipping to a separate location for priming, or priming on the paint line at the assembly plant. This amounts to high transportation and handling costs and a higher than normal scrap return rate.
U.S. Pat. No. 5,490,893, to Enlow et al., herein included by reference, discloses the use of a thermoformable conductive laminate to provide a conductive, paintable surface to the non-conductive surface and thereby overcome the problems of non-uniform conductivity and primer thickness. Enlow et al. form a matte release-coated casting sheet, then cast a thin film of an electrically conductive polymer so as to form a conductive primer coat. The conductive primer coat is transfer-laminated to a thermoformable plastic sheet to form a conductive face sheet which is thermoformed and bonded to the plastic substrate article. The article is then painted by applying the paint to the electrostatically paintable conductive face sheet. Paint is not applied to the non-conductive surface but to the conductive face sheet covering the non-conductive surface. Although Enlow et al. solve some problems, the dark base color of the thermoformable conductive laminate as well as the large number of steps in the process make this process uneconomical. Dark base colors generally make it difficult to obtain good paint coverage, especially with light colors. These disadvantages are compounded by the fact that a dark base color will easily show through when the paint layer is damaged such as when the article is scratched.
Accordingly, there remains a need in the art for a means to economically provide conductivity to non-conductive polymer surfaces with uniform conductivity and non-dark base color for electrostatic painting.
The present invention relates to a method for painting a non-conductive polymer article. The method comprises applying a conductive layer, such as a metal foil, adjacent to at least a portion of a second surface of the article so as to provide sufficient conductivity to enable electrostatic painting of a first surface of the article, and electrostatically painting the first surface.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description.
The above described drawbacks and deficiencies in the prior art are overcome by the invention of a method for electrostatic painting (coating) of a non-conductive polymer surface comprising applying a conductive layer on or adjacent to all or part of a side opposite the non-conductive polymer surface to be painted so as to provide sufficient conductivity to enable electrostatic painting of the non-conductive polymer surface. With the conductive layer in place the article can then be painted electrostatically. After painting, the conductive layer can optionally be removed without affecting the painted surface.
Unexpectedly it was found that when one side of a non-conductive part or layer (xe2x80x9cinner surfacexe2x80x9d) is brought fully or partially in contact with or disposed sufficiently close to a conductive layer, the opposite surface (xe2x80x9couter sidexe2x80x9d) becomes sufficiently conductive to be electrostatically paintable. Additionally, surfaces adjacent to the conductive layer can become sufficiently conductive to be electrostatically paintable. The non-conductive layer can have any thickness such that the surface of the non-conductive layer becomes sufficiently conductive to be electrostatically paintable. Thicknesses up to about 4 millimeters (mm) are typical, with a thickness of about 3 mm or less preferred, and about 2 mm or less especially preferred. It was also unexpectedly found that the conductive layer and the non-conductive layer do not have to be in full contact. Consequentially, the conductive layer is preferentially oriented, in relation to the non-conductive layer to be painted, sufficiently close and/or in physical contact to impart sufficient conductivity to the non-conductive layer to enable electrostatic painting thereof. Sufficient conductivity is defined as having a resistivity of about 1xc3x97105 ohm or less. It is typically sufficient for the conductive layer and non-conductive layer to be a distance of up to about 1.5 mm apart, with a distance of about 1 mm or less preferred, and direct physical contact between the non-conductive and conductive layers over at least a portion of the inner surface more preferred, and direct physical contact over substantially most of the inner surface especially preferred.
In addition to not requiring direct physical contact, the conductive layer does not need to be continuous. Small gaps and holes in the conductive layer up to about 5 mm2 or so in size do not affect the paint result, with smaller or no holes preferred. As with the proximity of the conductive layer to the non-conductive layer, the desired continuity of the conductive layer is based upon attaining the desired conductivity (e.g., resistivity about 1xc3x97105 ohm or less) substantially uniformly across the non-conductive layer.
Several kinds of conductive layers are useful, with conductive layers having a resistivity of about 200 ohms or less preferred, with those having a resistivity of about 40 ohms or less more preferred, and those having a resistivity of about 1 ohm or less especially preferred. Examples of useful conductive layers include metal foils, metal film laminates, metal wool, metal brushes and metal net, and the like, and combinations comprising at least one of the foregoing. Metal foil is defined as a sheet of metal with a thickness of about 300 microns or less. Metal foils and metal film laminates are preferred due to their ease in handling as well as low cost. Preferred metal film laminates include polyethylene/aluminum laminates and polyester/aluminum laminates with an aluminum thickness of about 4 microns or greater generally preferred. Especially useful polyesters include poly(butylene terephthalate) resin, for example, VALOX(copyright) resins available from GE Plastics, Pittsfield, Mass. Metal foils, such as aluminum foil, with a thickness sufficient to enable handling of the foil but sufficiently thin to be cost effective, e.g. about 5 microns to about 300 microns, are useful, with a thickness about 10 microns to about 100 microns preferred.
Useful non-conductive layers are those capable of attaining sufficient conductivity to enable electrostatic painting. Some useful materials include polymers and polymer blends with amide groups. Preferred materials are polyamide or polyamide blends such as compatibilized poly(arylene ether)/polyamide compositions (hereinafter PAE/PA). Polyamide and polyamide blends are well known in the art and are commercially available. Useful polyamide blends comprise at least about 10 weight percent polyamide. Quite unexpectedly it was found, as seen in the Examples below, only polymers containing polyamide were successfully electrostatically painted when the thickness of the conductive layer was about 16 micron or less.
In a first embodiment the conductive layer is brought fully or partially in contact with or disposed sufficiently close to a non-conductive layer. It is preferred for the conductive layer to be a foil or metal film laminate applied to the non-conductive layer. An adhesive may be used to maintain close proximity between the conductive and non-conductive layers. After the non-conductive layer is electrostatically painted the conductive layer may be removed or left in place.
In a second embodiment, the conductive/non-conductive layer system can comprise 2K molded conductive thermoplastic layers. 2K molding is a technique for producing an article with multiple layers of two or more different materials (e.g., two or three layers of two different materials). 2K molding can therefore produce an article with either a conductive layer and a non-conductive layer or a conductive layer sandwiched between two non-conductive layers. The non-conductive layer(s) will hereinafter be referred to as the skin and the conductive layer will be referred to as the core. Useful materials for the core include any injection moldable material with resistivity of about 200 ohms or less preferred and those having a resistivity of about 40 ohms or less more preferred and those having a resistivity of about 1 ohm or less especially preferred. A preferred core material is a conductive PAE/PA. Especially useful as the core material is a conductive PAE/PA such as Noryl GTX(copyright) VP081 resin, available from GE Plastics. Useful skin materials are injection moldable materials capable of attaining sufficient conductivity to enable electrostatic painting. Preferred materials are polyamide and PAE/PA as described in the first embodiment. Especially preferred are Noryl GTX 944 and Noryl GTX 964 resins, PAE/PA""s with different viscosities available from GE Plastics.
The juxtaposition of the conductive layer (core) adjacent to the non-conductive layer in 2K molding allows the non-conductive layer (the skin) to be electrostatically painted. Two advantages of 2K molding are the desirable physical properties of the non-conductive layer such as ductility are maintained and the conductive layer is provided in the same step as the molding of the article. 2K molding is especially useful in articles which must be painted on more than one side. With the conductive layer within the article, all sides can be electrostatically painted without repeated conductive layer applications.
The non-conductive layer can be comprised of any polymer composition which can attain the desired conductivity when disposed next to or in physical contact with a conductive layer. Some useful materials include polymers,and polymer blends with amide groups, notably polyamide, poly(arylene ether)/polyamide compositions and the like. Noryl GTX 944 and Noryl GTX 964 resins, PAE/PA""s available from GE Plastics, are particularly useful.
Once the non-conductive polymer is formed into an article by any conventional technique and the conductive layer is disposed on or adjacent to one or more surfaces so as to provide sufficient conductivity to enable electrostatic painting of the opposite surface(s). Once the opposite surface has been painted electrostatically, the conductive layer can be removed. For example, aluminum foil, preferably with a thickness greater than about 5 microns, is applied on or adjacent to the inner surface of an article made of non-conductive PAE/PA. The article is then painted electrostatically and the foil is removed after painting is complete.